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Similar questions to this have been asked, but I wasn't able to find one that answers this specifically.

Say that you have resistance to slashing damage as well as resistance to fire damage (no caveats to nonmagical damage or not). You are fighting someone who has a flaming greataxe (just an enchanted item) that deals 1d12+STR slashing damage, plus 1d6 fire damage on hit and they hit you.

How do you calculate the damage?

  • (1d12+1d6+STR)/2
  • (1d12/2) + (1d6/2) + STR
  • (1d12+STR/2) + (1d6/2)
  • something else?
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    \$\begingroup\$ What "flaming greataxe" are you talking about? If it is a homebrew item, you should ask the author. Normally a weapon doesn't deal "1d12 (axe) + 1d6 (fire) + modifier" damage. More standard description would be "1d12+STR slashing damage, plus 1d6 fire damage on hit", which is pretty straightforward. \$\endgroup\$ – enkryptor Oct 11 '17 at 16:47
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    \$\begingroup\$ I updated the question to include STR instead of modifier and how you wrote the description to be clearer. And I guess it is homebrew, it's just an axe a bandit boss the party fought that I came up with. \$\endgroup\$ – bubbajake00 Oct 11 '17 at 16:54
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    \$\begingroup\$ [Related] What part of the damage is halved/doubled for Damage Resistance/Vulnerability? \$\endgroup\$ – SevenSidedDie Oct 11 '17 at 17:53
  • \$\begingroup\$ By the distributive property, your first and third options (dividing the sum versus dividing each addend) are equivalent. Your second option doesn't make any sense, since the STR modifier is part of the damage and therefore should be subject to resistance. \$\endgroup\$ – Kyle Strand Oct 11 '17 at 21:06
  • \$\begingroup\$ @KyleStrand I had the 2nd one based on how Criticals are calculated, where it is your damage is doubled but not your modifier. In that circumstance it is part of the damage, but not part of the weapon damage itself. Basically, your weapon strikes true but you are still hitting as hard as you were before. Of course thematically, it can be different. \$\endgroup\$ – bubbajake00 Oct 11 '17 at 21:10
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You would calculate the resistance to each damage separately. The modifier should also have a damage type, and you would include it with the dice roll for calculating resistance.

For example, with a melee weapon like a greataxe where you apply your strength to its slashing damage you would calculate \$(1{\rm d}12 + \text{Str Mod})\div2\;\$ (rounding down) slashing damage. And then calculate \$1{\rm d}6 \div 2\$ (also rounding down) fire damage.

So your final formula would be:

$$\left\lfloor \frac{1{\rm d}12 + \text{Str Mod}}{2} \right\rfloor + \left\lfloor \frac{1{\rm d}6}{2} \right\rfloor$$

(Those lines on the sides being the floor/round down symbols.)

RAW would probably be as described in this answer. They reference:

Damage Resistance: If a creature or an object has resistance to a damage type, damage of that type is halved against it.

Damage Vulnerability: If a creature or an object has vulnerability to a damage type, damage of that type is doubled against it.

Resistance and then vulnerability are applied after all other modifiers to damage. For example, a creature has resistance to bludgeoning damage and is hit by an attack that deals 25 bludgeoning damage. The creature is also within a magical aura that reduces all damage by 5. The 25 damage is first reduced by 5 and then halved, so the creature takes 10 damage.

(D&D 5e SRD, Pg 97)

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    \$\begingroup\$ Thank you for finding the RAW for this and going into all the detail. \$\endgroup\$ – bubbajake00 Oct 11 '17 at 18:11
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    \$\begingroup\$ Ah! Xanathar's actually has a blurb on this! As I understand, nothing changes with this answer but I will update this answer later with the new excerpt from the book. \$\endgroup\$ – Rob Rose Nov 22 '17 at 21:00
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RAW says resistance is only applied once

PHB 197:

Multiple instances of resistance or vulnerability that affect the same damage type count as only one instance. For example, if a creature has resistance to fire damage as well as resistance to all nonmagical damage, the damage of a nonmagical fire is reduced by half against the creature, not reduced by three-quarters.

Therefore, if your greataxe attack is a single damage roll that's both fire and slashing damage, the resistance is only applied once.

On the other hand, if it deals the two damages separately, then only the specific parts of the damage that is resisted would be halved. I couldn't find a reference for this in the books, but for something like Meteor Swarm, which deals both bludgeoning and fire damage, fire resistance would only halve the fire damage and not the bludgeoning.

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  • \$\begingroup\$ Do you mean if Meteor Storm were used to hit a creature with fire resistance and not bludgeoning resistance? \$\endgroup\$ – Rob Rose Oct 11 '17 at 17:28
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    \$\begingroup\$ You seem to have missed the "affect the same damage type" qualifier in your quoted text. Fire and slashing are not the same damage type. \$\endgroup\$ – Kyle Strand Oct 11 '17 at 21:04
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    \$\begingroup\$ I'm not aware of anything in 5e-as-written that deals damage with more than one type (such as "fire and slashing"). The Flametongue sword deals normal weapon damage and an additional 2d6 fire damage. Flamestrike does 4d6 fire and 4d6 radiant. So if you have resistance to fire, you halve the 4d6 fire damage but not the 4d6 radiant. If you have resistance to fire and radiant, you halve both of them. That said, you're not wrong in your interpretation of the rule -- if you have resistance to an instance of damage for more than one reason, regardless of why, you halve it once only. \$\endgroup\$ – Darth Pseudonym Oct 13 '17 at 20:47

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